Re: [PATCH] KVM: arm64: Drop mte_allowed check during memslot creation

Suzuki K Poulose
2025-02-24T10:32:04Z

On 24/02/2025 09:39, Aneesh Kumar K.V (Arm) wrote:
> Before commit d89585fbb308 ("KVM: arm64: unify the tests for VMAs in
> memslots when MTE is enabled"), kvm_arch_prepare_memory_region() only
> rejected a memory slot if VM_SHARED was set. This commit unified the
> checking with user_mem_abort(), with slots being rejected if either
> VM_MTE_ALLOWED is not set or VM_SHARED set. A subsequent commit
> c911f0d46879 ("KVM: arm64: permit all VM_MTE_ALLOWED mappings with MTE
> enabled") dropped the VM_SHARED check, so we ended up with memory slots
> being rejected if VM_MTE_ALLOWED is not set. This wasn't the case before
> the commit d89585fbb308. The rejection of the memory slot with VM_SHARED
> set was done to avoid a race condition with the test/set of the
> PG_mte_tagged flag. Before Commit d77e59a8fccd ("arm64: mte: Lock a page
> for MTE tag initialization") the kernel avoided allowing MTE with shared
> pages, thereby preventing two tasks sharing a page from setting up the
> PG_mte_tagged flag racily.
>
> Commit d77e59a8fccd ("arm64: mte: Lock a page for MTE tag
> initialization") further updated the locking so that the kernel
> allows VM_SHARED mapping with MTE. With this commit, we can enable
> memslot creation with VM_SHARED VMA mapping.
>
> This patch results in a minor tweak to the ABI. We now allow creating
> memslots that don't have the VM_MTE_ALLOWED flag set. If the guest uses
> such a memslot with Allocation Tags, the kernel will generate -EFAULT.
> ie, instead of failing early, we now fail later during KVM_RUN.
>
> This change is needed because, without it, users are not able to use MTE
> with VFIO passthrough (currently the mapping is either Device or
> NonCacheable for which tag access check is not applied.), as shown
> below (kvmtool VMM).
>
> [ 617.921030] vfio-pci 0000:01:00.0: resetting
> [ 618.024719] vfio-pci 0000:01:00.0: reset done
> Error: 0000:01:00.0: failed to register region with KVM
> Warning: [0abc:aced] Error activating emulation for BAR 0
> Error: 0000:01:00.0: failed to configure regions
> Warning: Failed init: vfio__init
>
> Fatal: Initialisation failed
>
> Signed-off-by: Aneesh Kumar K.V (Arm) <aneesh.kumar@kernel.org>

Tested-by: Suzuki K Poulose <suzuki.poulose@arm.com>